The technical field generally relates to SCR aftertreatment systems for internal combustion engines. Presently known SCR aftertreatment systems for engines suffer from several drawbacks, including control with an unknown amount of NH3 stored on the SCR catalyst, and difficulty managing aging and degradation of the catalyst. Further, current SCR aftertreatment systems are highly reliant on NOx input data, which depend upon NOx models or a NOx sensor. Accurate NOx models are difficult to establish for transiently operated engine systems, and NOx sensors are expensive. Therefore, further technological developments are desirable in this area.